1. Field of the Invention
The present invention relates to a servo control apparatus, servo control method, optical disc apparatus and video camera recorder, and is preferably applied to an optical disc apparatus, for example.
2. Description of Related Art
An optical disc apparatus is usually designed to record on an optical disc, or a recording medium, information such as music and video data. In addition to that, the optical disc apparatus is used to read out the information from the optical disc. This type of apparatus has become widely used.
To record the information, the optical disc apparatus rotates the optical disc and emits an optical beam that is then focused on a target track on the optical disc. To reproduce the information, the optical disc apparatus detects the reflected optical beam.
For example, as shown in FIG. 9, an optical disc apparatus 1, under the control of a servo control section 2, emits from a laser diode 4 of an optical pickup 3 an optical beam L1 which then passes through a collimator lens 5, a polarization beam splitter 6, a raising mirror 7, a quarter wave length plate 8 and an objective lens 9 to be focused on a signal recording surface 10A of the optical disc 10.
In the optical disc apparatus 1, the optical beam L2, reflected on the signal recording surface 10A of the optical disc 10, passes through the objective lens 9, the quarter wave length plate 8 and the raising mirror 7. This reflection optical beam L2 subsequently gets into the polarization beam splitter 6.
The reflection optical beam L2 is then reflected by a polarization surface 6A of the polarization beam splitter 6. The reflection optical beam L2 subsequently passes through a collimator lens 11 and a hologram 12 and then reaches a photodetector 13. In the optical disc apparatus 1, the photodetector 13 detects the reflection optical beam L2. The optical disc apparatus 1 then produces a detection signal based on the intensity of the reflection optical beam L2 and then performs a predetermined signal process to reproduce information.
In the optical disc apparatus 1, the distance between the objective lens 9 and the optical disc 10 may vary. This is partly due to the unique characteristics of each optical disc 10 and the trembling of the surface of the optical disc 10. In other words, a focused focal location JF, a location on the signal recording surface 10A of the optical disc 10 on which the optical beam L1 through the objective lens 9 is focused may vary according to the unique characteristics of each optical disc 10 or the position of the signal recording surface 10A of the optical disc 10.
Accordingly, the optical disc apparatus 1 performs a servo control process to keep the distance between the objective lens 9 and the optical disc 10 at a constant level: an actuator 14 moves the objective lens 9 such that the objective lens 9 gets close to or away from the optical disc. Thus, the optical beam L1 is appropriately focused on the signal recording surface 10A of the optical disc 10 (this process is a so-called focusing).
Specifically, the optical disc apparatus 1 uses the method of Spot Size Detection (SSD) for the focusing process. As shown in FIG. 10A, the hologram 12 of the optical disc apparatus 1 divides the reflection optical beam L2 into a plus first order light L2A, a zero order light L2C and a minus first order light L2B. The focal distances of these light beams L2A, L2C and L2B are different from each other. Those light beams L2A, L2C and L2B are supplied to detection areas 15, 16 and 17 on the photodetector 13.
As shown in FIG. 10B, each of the detection areas 15 and 16 is divided into three sections, each of which photo-electrically converts the plus first order light L2A and the minus first order light L2B into detection signals S1A to S1C and S2A to S2C. The detection areas then supplies the detection signals S1A to S1C and S2A to S2C to the control section 2.
The servo control section 2 of the optical disc apparatus 1 controls a focus error signal detection circuit 21 to perform a calculation process using the detection signals S1A to S1C and S2A to S2C to produce a focus error signal SFE. The calculation is expressed as follows:SFE=S2A+S2B+S1C−(S1A+S1B+S2C)  (1)
The servo control section 2 subsequently controls an analog-to-digital (A/D) converter 22 to convert the analog focus error signal SFE into a digital focus error data DFE and supplies the focus error data DFE to a focus servo control circuit 23.
The focus servo control circuit 23 generates, based on the focus error data DFE, control data DC which is used to control the actuator 14. A digital-to-analog (D/A) converter 24 converts the control data DC into an analog control signal SC and then supplies the control signal SC to an actuator drive circuit 25.
The actuator drive circuit 25 produces, based on the control signal SC, an actuator drive signal SA and then supplies the actuator drive signal SA to the actuator 14. The optical pickup 3 controls the actuator 14 in accordance with the actuator drive signal SA.
On the other hand, the focus servo control circuit 23 is designed to read out various data from a nonvolatile memory 26.
The photodetector 13 is located such that when the objective lens 9 of the optical disc apparatus 1 is focused on the signal recording surface 10A of the optical disc 10 (or on the focused focal location JF), the zero order light L2C is focused on the detection area 17 as shown in FIG. 10A. At this time, the focus position of the plus first order light L2A is behind the photodetector 13 (or the upper side of the diagram) while the focus position of the minus first order light L2B stops short of the photodetector 13 (or the lower side of the diagram).
If the objective lens 9 of the optical disc apparatus 1 does not align with the focused focal location JF, the focus positions of the zero order light L2C, plus first order light L2A and minus first order light L2B also change in accordance with the amount of the misalignment (i.e., upward or downward in the diagram). Accordingly, the intensity of the plus first order light L2A and minus first order light L2B, received by the detection areas 15 and 16, may change.
This means that the signal level of the focus error signal SFE change in accordance with the distance between the objective lens 9 and the optical disc 10: this represents a substantially S-shaped characteristic curve Q1 as shown in FIGS. 11A and 11B.
According to the equation (1) or the SSD method and the position of the photodetector 13, when the objective lens 9 is located at the focused focal location JF which allows the optical beam L1 to be focused on the signal recording surface 10A of the optical disc 10, the focus error signal SFE has a value of zero as shown in FIG. 11B.
The characteristic curve Q1 of the focus error signal SFE changes linearly around the focused focal location JF while the characteristic curve Q1 changes nonlinearly as it gets away from the focused focal location JF, and becoming close to zero.
Therefore, one can assume that only when the objective lens 9 is close to the focused focal location JF, the signal level of the focus error signal SFE is substantially proportional to the distance between the objective lens 9 and the focused focal location JF (hereinafter, the linear portion of the characteristic curve Q1 is also referred to as a “detection area AD”).
The nonvolatile memory 26 of the servo control section 2 has memorized a coefficient COE which represents the correlation between the signal level of the focus error signal SFE and the distance between the objective lens 9 and the focused focal location JF.
When being located around the focused focal location (or inside the detection area AD), the servo control section 2 of the optical disc apparatus 1 calculates the distance between the objective lens 9 and the focused focal location JF using the coefficient COE and generates the actuator drive signal SA to make the focus error signal SFE (or the focus error data DFE) close to zero. The actuator 14 operates in accordance with the actuator drive signal SA and then moves the objective lens 9 toward the focused focal location JF (Servo Control).
In this manner, the control section 2 performs feedback control: the control section 2 generates the focus error signal SFE based on the detection signals S1A to S1C and S2A to S2C and then produces the actuator drive signal SA based on the focus error signal SFE to control the actuator 14.
Hereinafter, a closed loop including the photodetector 13, the servo control section 2 and the actuator 14 is also referred to as a “focus servo control system”. In addition, the feedback control, a process performed by the actuator 14 based on the focus error signal SFE is also referred to as “focus servo control”.
By the way, when the optical disc 10 is inserted or ejected from the optical disc apparatus 1, the servo control section 2 moves the objective lens 9 away from the optical disc 10 to the outside of the detection area AD to prevent contact and damage between the optical disc 10 and the objective lens 9.
Accordingly, the servo control section 2 controls, when the optical disc 10 is inserted, the actuator 14 to move the objective lens 9 toward the optical disc 10 such that the objective lens 9 is located substantially close to the focused focal location JF (or inside the detection area AD). In this manner, the servo control section 2 performs such a process as the so-called pull-in operation before starting the focus servo control process.
Specifically, the servo control section 2 uses the actuator drive circuit 25 to control the actuator 14. In addition, while detecting the focus error signal SFE (or the focus error data DFE) through the focus error signal detection circuit 21, the servo control section 2 moves the objective lens 9 toward the optical disc 10 at a constant speed.
At a time instant 0 when the signal level of the focus error signal SFE drops, as shown in FIG. 12, to below a predetermined value which is less than zero after rising from zero to a certain level, the control section 2 determines that the objective lens 9 has just passed through the focused focal location JF, or that the objective lens 9 is substantially located inside the detection area AD. In response to that, the control section 2 starts the focus servo control process of the actuator 14.
By the way, there is a demand for the control section 2 to start the playback and recording of the optical disc 10 as soon as possible by completing the pull-in operation in a short period of time. That is, it is desirable that as shown in FIG. 12 (dotted lines) for example, the objective lens 9 should be focused on the location JF as soon as possible after the start of the focus servo control process (i.e. after the time instant 0). This means a good transient response is desirable.
FIG. 13 is a schematic block diagram illustrating a focus servo control system of the optical disc apparatus 1.
A general type of a control circuit of an optical disc apparatus usually uses a second order filter including a low frequency emphasis filter and a high frequency phase advance filter. Accordingly, a control circuit 30K of the focus servo control system 30 has two state variables: a low-pass filter value fc1 and a high-pass filter value fc2. On the other hand, a controlled section 30P, controlled by the control circuit 30K, is equivalent to the actuator 14, holding two state variables, i.e., a location x and a velocity v.
As shown in FIG. 12, the optical disc apparatus 1 starts the focus servo control when the moving objective lens 9 is not at the focused focal location JF. The following describes the situation in which at the time instant 0 when the focus servo control is initiated, the location x and velocity v of the controlled section are not zero (i.e., Initial value response).
FIG. 14 is a block diagram illustrating the state space of a focus servo control system, which is equivalent to the system shown in FIG. 13. The state of the focus servo control system 31 can be represented by four state variables (a low-pass filter value fL, a high-pass filter value fH, a location x and a velocity v) as follows:
                              X          ->                =                  (                                                                      f                                      c                    ⁢                                                                                  ⁢                    1                                                                                                                        f                                      c                    ⁢                                                                                  ⁢                    2                                                                                                      x                                                                    v                                              )                                    (        2        )            
Using inverse Laplace transform, an output y(t) of the focus servo control system 31 at a time instant t (later than the time instant 0 [FIG. 12]) is also represented as follows:y(t)=L−1[C·(sI−A)−1]{right arrow over (x)}(0)  (3)where the output y(t) is a value representing the position of the objective lens 9 with respect to the focused focal location JF.
A state X(0) of the equation (3) represents the initial value of the focus servo control system 31 as a whole. It is evident that the output y(t) be changed in accordance with the state X(0).
FIGS. 15A to 15D are schematic diagrams illustrating each response of the state variables (the low-pass filter value fL, the high-pass filter value fH, the location x and the velocity v) of the state X(0) of the focus servo control system 31. When being superimposed, they are represented as an output y(t) as shown in FIG. 15E.
Generally, the low-pass filter value fL(0) and the high-pass filter value fH(0), the initial values of the control circuit 31K of the focus servo control system 31 are zero.
By contrast, Patent Document 1 (Jpn. Pat. No. 2685622 [Pages 4 to 5]), for example, discloses an initial value compensation method capable of obtaining a good response: setting an appropriate value as the initial value of the control circuit 31K, the output y(t) can converge in a short period of time.
FIG. 16 shows the configuration of the focus servo control system of the optical disc apparatus 1, including digital filters: it is taken into consideration that the focus servo control circuit 23 of the control section 2 performs digital calculation processes.
In the focus servo control system 32 shown in FIG. 16, the low-pass filter value fL(0) and the high-pass filter value fH(0) are respectively supplied to a register 32RL of a low-pass filter section 32L and a register 32RH of a high-pass filter section 32H as the initial values.
In the initial value compensation method, the initial values of the control circuit (the low-pass filter value fL(0) and the high-pass filter value fH(0)) are represented by a function using the initial values of the controlled section (the location x(0) and the velocity v(0)) as follows:
                              (                                                                                          f                                          c                      ⁢                                                                                          ⁢                      1                                                        ⁡                                      (                    0                    )                                                                                                                                            f                                          c                      ⁢                                                                                          ⁢                      2                                                        ⁡                                      (                    0                    )                                                                                )                =                              α            ⁡                          (                                                                                          x                      ⁡                                              (                        0                        )                                                                                                                                                        v                      ⁡                                              (                        0                        )                                                                                                        )                                =                                    (                                                                                          k                      11                                                                                                  k                      12                                                                                                                                  k                      21                                                                                                  k                      22                                                                                  )                        ⁢                          (                                                                                          x                      ⁡                                              (                        0                        )                                                                                                                                                        v                      ⁡                                              (                        0                        )                                                                                                        )                                                          (        4        )            
In this case, the location x of the controlled section can be calculated from the focus error signal SFE (or the focus error data DFE) and the coefficient COE. The velocity v can be calculated in the following manner: the location x of the controlled section is continuously calculated over time, and the velocity v is calculated based on its variation and the elapsed time.
In fact, the location x(0) and the velocity v(0), the initial values of the controlled section can be assumed as the location x and velocity v of the objective lens 9 at the time instant 0, which are calculated based on the focus error data DFE.
In addition, a matrix α (i.e., coefficients K11, k12, k21 and k22) of the equation (4) is determined based on the response characteristics of the actuator 14 of the optical disc apparatus 1, the weight of the objective lens 9 and the like. It can be calculated using design methods such as an evaluation value function minimum method or a zero point specification method.
In the initial value compensation method, using the location x(0) and the velocity v(0) at a timing when the focus servo control is initiated, the calculation of the equation (4) is performed to obtain the low-pass filter value fL(0) and the high-pass filter value fH(0) to present the best output y(t).